In the field of LED drivers resonant converters like LLC converters are commonly known for driving LEDs. The resonant converter control the output power provided to the load by switching two controllable switches and by providing an alternating drive voltage to an electromagnetic transformer. The energy transferred by the resonant converter is proportional to the energy change in a capacitor between the two switching states of the controllable switches. The energy provided to the load is controlled by switching the controllable switches.
The light emitted from the LED units can also be used for transmitting information such as a light identity number, which is invisible for the user. The so-called coded light can be implemented as a modulated output power of the resonant converter, e.g. the LLC converter. A method for modulating the light emission of a lighting device is e.g. known from WO 2008/001262 A1.
A further possibility to transmit information by means of driver devices for driving LED units is to transmit coded sound similar to the above-mentioned coded light. The coded sound has the advantage that the receiver of the coded sound does not need to be inline of sight to the emitter and, further, if the coded sound is in the ultrasound range, it is not audible for the user.
The power output of the resonant converter is usually controlled by varying the duty cycle of the controllable switches as disclosed by US 2011/0164437 A1. Alternatively, the output power of the resonant converters can be controlled by changing the switching frequency of the switches as e.g. disclosed by U.S. Pat. No. 7,313,004 B1.
A key problem of the transmission of the coded light or the coded sound is that the transmission is perceptible for the user either due to a perceptible flicker of the emitted light or due to the sound when the information packet is transmitted. Usually the perceptible flicker is minimized by using certain coding schemes which have a mean value of zero.
Since highly efficient power converter like resonant converter are highly non-linear regarding their frequency control, it is difficult to modulate the amplitude of an emitted electrical power for powering a load and for providing a respectively coded light or sound.